1. Field of the Invention
The present invention relates to an OLED (organic light emitting diode) panel obtained by forming an OLED on a substrate and sealing the OLED between the substrate and a cover member. The invention also relates to an OLED module in which an IC including a controller, or the like, is mounted to the OLED panel. In this specification, ‘light emitting device’ is the generic term for the OLED panel and for the OLED module. Electronic equipment using the light emitting device is also included in the present invention.
2. Description of the Related Art
Being self-luminous, OLEDs eliminate the need for a backlight that is necessary in liquid crystal display devices (LCDs), and thus they are most suitable when manufacturing thinner devices. Also, the self-luminous OLEDs are high in visibility and have no limit in terms of viewing angle. These are the reasons for the attention that light emitting devices using the OLEDs are receiving in recent years as display devices to replace CRTs and LCDs.
An OLED has a layer containing an organic compound (organic light emitting material) that provides luminescence (electroluminescence) when an electric field is applied (the layer is hereinafter referred to as organic light emitting layer), in addition to an anode layer and a cathode layer. Luminescence obtained from organic compounds is classified into light emission upon return to the base state from singlet excitation (fluorescence) and light emission upon return to the base state from triplet excitation (phosphorescence). A light emitting device according to the present invention can use one or both types of the light emission.
In this specification, all the layers that are provided between an anode and a cathode together make an organic light emitting layer. Specifically, the organic light emitting layer includes a light emitting layer, a hole injection layer, an electron injection layer, a hole transporting layer, an electron transporting layer, etc. A basic structure of an OLED is a laminate of an anode, a light emitting layer, and a cathode layered in this order. The basic structure can be modified into a laminate of an anode, a hole injection layer, a light emitting layer, and a cathode layered in this order, or a laminate of an anode, a hole injection layer, a light emitting layer, an electron transporting layer, and a cathode layered in this order.
The problem in putting a light emitting device into practice is lowering in luminance of OLED which accompanies degradation of its organic light emitting material.
Organic light emitting materials are weak against moisture, oxygen, light, and heat, which accelerate degradation of the organic light emitting materials. Specifically, the rate of degradation of an organic light emitting material depends on the structure of a device for driving the light emitting device, characteristics of the organic light emitting material, materials of electrodes, manufacture process conditions, how the light emitting device is driven, etc.
Even when the voltage applied to the organic light emitting layer is constant, the luminance of the OLED is lowered as the organic light emitting layer degrades, and an image displayed therefore becomes unclear. In this specification, a voltage applied to an organic light emitting layer from a pair of electrodes is called an OLED drive voltage (Vel).
When an image is displayed in color by using three types of OLEDs that respectively emit red (R) light, green (G) light, and blue (B) light, different organic materials are used to form organic light emitting layers of OLEDs of different colors. Accordingly, the rate of degradation of organic light emitting layer may vary between OLEDs of different colors. Then the difference in luminance between OLEDs of different colors will be noticeably large as time passes, making it impossible for the light emitting device to display an image in desired colors.
The temperature of organic light emitting layer is influenced by the outside temperature and heat generated from the OLED panel itself. Generally, the amount of current flowing in an OLED varies depending on the temperature. FIG. 26 shows a change in voltage-current characteristic of an OLED when the temperature of its organic light emitting layer is changed. With the voltage kept constant, the OLED drive current is increased as the temperature of the organic light emitting layer rises. Since the OLED drive current is in proportion to the OLED luminance, the luminance of the OLED becomes higher as the OLED drive current becomes larger. Since a change in temperature of the organic light emitting layer thus causes a change in OLED luminance, displaying an image in desired gray scales is difficult and current consumption of the light emitting device is increased accompanying a temperature rise.
Generally, temperature change brings varying degrees of changes in OLED drive current to different types of organic light emitting materials and, therefore, in color display, the luminance could be changed by temperature change differently for OLEDs of different colors. It is impossible to obtain desired colors when OLEDs of different colors lose their luminance balance.